Overview:Diphtheria
is a respiratory infection caused by the
Gram-positive, non-motile, rod-shaped bacillus,
Corynebacyerium diphtheriae (Figure
1). This
organism ranges in size from 0.5 um
(micrometres) to 5 um in length; however, the
average bacterium is typically 1 um.
C. diphtheriae is aerobic and is
divided into four distinct subspecies that
differ slightly in their colonial morphology and
biochemical properties such as the ability to
metabolize certain nutrients. The four
recoginzed subspecies are: C. diphtheriae
mitis, C. diphtheriae intermedius,
C. diphtheriae gravis, and C. diphtheriae
belfanti. Toxin-production occurs
only when the bacillus is itself infected (lysogenized)
by a bacteriophage carrying the genetic
information for the toxin (tox gene)
(Dover et al., 2003) (Figure
2).
All may be
toxigenic (and therefore cause diphtheria) or
non-toxigenic, depending on the genes they
possess. Furthermore, this organisms may also
contain inclusion bodies within its cytoplasm,
known as metachromatic granules, which are
composed of inorganic polyphosphates. These
inclusion bodies serve as energy reserves, and
are not membrane bound.

Figure 1. A photomicrophage of
Corynebacterium diphtheriae [1200 X].

Figure 2. Bacteriophages are
viruses that transfect bacteria with genomic
material. They can be used in a laboratory to
introduce genes into bacterial cells by
transduction (transfer of genes from one
bacteria to another).

Diphtheria occurs worldwide and is transmitted
human to human by airborne droplets of
respiratory secretions. Infection usually begins
in the throat (pharynx) within two to four days
after contact. Inflammation and patches of
exudate appear in the epithelial cells of the throat
and tonsils. Localized swelling
may result in the classic 'bull neck' appearance
(Figure 3). The patches coalesce to form a
sticky, tough diphtheritic membrane, and this is
mainly due to the bacteria's ability to produce
pili (fimbriae), which allow it to adhere to
cellular surfaces. Extension
of the membrane in the respiratory passages
(trachae) reduces air flow and can
result in suffocation, if not treated with
antibiotics such as penicillin. The membrane consists of
fibrin, necrotic tissue, blood cells and
bacterial colonies. The name
Diphtheria is derived from the Greek
word for leather, referring to this tough
membrane.

Figure 3. This child with
diphtheria presented with a characteristic
swollen neck, sometimes referred to as bull
neck.

Diphtheria is an acute bacterial disease
involving primarily the tonsils, pharynx,
larynx, nose, skin, and at times other mucous
membranes. The mucosal lesion is marked by a
patch or patches of an adherent grayish membrane
with a surrounding inflammation.

The bacilli remain on the respiratory mucosa and
secrete a lethal extoxin, known simply as
diphtheria toxin. This toxin is
responsible for the serious consequences of the
disease; in fact, 100 ng (nanograms) per kilogram body weight is
lethal. The toxin inhibits protein synthesis by
inactivating the elongation factor (EF2)
in ribosomes during translation. It does this by
ADP-ribosylating the unusual amino acid
diphthamide found in EF2 (Figure
4). In this way, it acts as a RNA translational
inhibitor. The exotoxin A of Pseudomonas
aeruginosa uses a similar mechanism of
action. Recall that EF2 catalyzes the
translocation of the tRNA and mRNA down the
ribosome at the end of each round of polypeptide
elongation, orchestrating the events of
translational elongation during protein
synthesis. The circulatory system carries the
toxin throughout the body. Mortality from
diphtheria can range from 30 to 50%, frequently
occurring in children. Suffocation is the
primary cause of death, followed by myocarditis
and polyneuritis

due to the toxicity of the diphtheria
exotoxin.

Figure 4. ADP-ribosylation of
EF2 prevents the translation
of mRNA during protein synthesis. Here, tRNA7
will not be able to transfer over the amino acid
it is carrying to the newly formed petide due to
the steric hindrance caused by the ADP molecule.

A toxoid vaccine is the best strategy for the
diphtheria vaccine. In fact, the diphtheria
toxoid is one of the most effective vaccines. It
has been administered to children in the
trivalen (DPT) (Diphtheria, pertussis, and
tetanus) vaccine since 1955. Booster
inoculations every decade are recommended. In
1922, Ramon developed the current toxoid
vaccine. He demonstrated that the exotoxin
irreversibly lost its toxicity after treatment
with heat and formaldehyde.